1. Field of the Invention
The present invention generally relates to a metal/ceramic bonding article having a ceramic substrate and a metal plate which is bonded to the ceramic substrate via a brazing filler metal. More specifically, the invention relates to a metal/ceramic bonding article on which parts, such as semiconductor parts, are mounted and which is used for a power module or a Peltier element module.
2. Description of the Prior Art
In a typical method for producing a ceramic circuit board for a power module or for mounting a semiconductor thereon, a metal plate and a ceramic substrate are first bonded to each other For example, there is industrially utilized the direct bonding method for arranging a copper plate on a ceramic substrate so as to allow the copper plate to directly contact the ceramic substrate and for heating the copper plate and the ceramic substrate in an inert gas to bond the ceramic substrate and the copper plate to each other. There is also industrially utilized the brazing and soldering method for arranging a copper plate on a ceramic substrate via a brazing filler metal containing an active metal, such as Ti, Zr or Hf, and for heating them in a vacuum to bond the ceramic substrate and the copper plate to each other. In the brazing and soldering method, the active metal concerns the bonding of the ceramic substrate to the metal plate, and the ceramic substrate reacts with the brazing filler metal to form a reaction product. It is generally considered that a brazing filler metal reacts with a ceramic substrate of an oxide, such as Al2O3, to form an oxide of an active metal, reacts with a ceramic substrate of a non-oxide, such as AlN or Si3N4, to form a nitride of an active metal, and reacts with a ceramic substrate of a carbide, such as SiC, to form a carbide of an active metal, so that the product bonds the ceramic substrate to the copper plate. That is, the brazing filler metal layer after bonding comprises a layer mainly containing the metal, and a layer mainly containing an interface product of the interface between the brazing filler metal and the ceramic substrate.
As a method for forming a predetermined circuit shape by patterning after bonding a metal plate, such as a copper plate, for a circuit or radiation, there is the etching method also utilized for a printed circuit board or the like. This method is widely utilized since it is easy to obtain a fine pattern and it is possible to relatively simply cope with the change of a circuit design. In this method, for example, a mixed solution of iron chloride or copper chloride, hydrochloric acid and hydrogen peroxide is usually used as an etchant for a metal plate, such as a copper plate. In the case of the above described direct bonding method, this etchant can carry out etching and patterning without causing problems since it is possible to ignore reaction products. However, in the case of the brazing and soldering method, this etchant can dissolve the metal plate, but it can not dissolve the brazing filler metal and a reaction product of the brazing filler metal with the ceramic substrate (the general term for the brazing filler metal and the reaction product will be hereinafter referred to as a xe2x80x9cbrazing filler metal and so forthxe2x80x9d), so that the brazing filler metal and so forth remain between circuit patterns and/or on the edge face of the substrate. Since the brazing filler metal and so forth are conductors, it is not possible to satisfy basic characteristics of a circuit board to isolate the circuit patterns from each other and/or the surface and reverse of the board from each other. As a method for removing the brazing filler metal and so forth, there is known a method for using hydrofluoric acid alone or a mixed acid of hydrofluoric acid and at least one inorganic acid selected from the group consisting of nitric acid, sulfuric acid and hydrochloric acid, or using a solution containing aqua regia, sodium hydroxide and/or potassium hydroxide, to treat and remove the brazing filler metal and so forth (see Japanese Patent No. 2,594,475). There is also known a method for treating the brazing filler metal and so forth with a solution containing a hydrogen halide and/or an ammonium halide, and then, treating them with a solution containing an inorganic acid and hydrogen peroxide, to remove the brazing filler metal and so forth (see Japanese Publication No. 7-36467).
On the metal circuit portion of a metal/ceramic bonding substrate patterned by the above described processes, nickel plating, nickel alloy plating, gold plating or preservation is carried out in accordance with its purpose.
Moreover, chip parts, such as semiconductor parts, are mounted thereon by soldering or the like to be used as a power module or a Peltier element module.
In recent years, power modules and Peltier element modules are used in severer environment, and parts used for them are required to have high reliability. In particular, parts used as automotive parts or used outdoors are required to improve thermal shock resistance. On the other hand, for example, in some of metal/ceramic bonding substrates wherein a metal is bonded to a ceramic substrate via a brazing filler metal, characteristics are further improved by devising the sectional shape of the edge portions of a circuit pattern.
In order to enhance reliability against thermal shock and so forth by means of a brazing filler metal, it is known that protrusion of the brazing filler metal from edge portions of a metal plate is effective in relaxation of the thermal stress caused by the difference in coefficient of thermal expansion between metal and ceramic in the bonding of metal to ceramic. For example, Japanese Patent Laid-Open No. 10-326949 has proposed a substrate having a structure wherein the difference between the dimensions of the bottom and top faces of the peripheral edge portion of a metal circuit plate is in the range of from 50 xcexcm to 100 xcexcm (this difference is the distance between a plane perpendicular to the principal plane of a metal plate at one end of the bottom face of the metal plate and a plane perpendicular to the principal plane of the metal plate at one end of the top face of the metal plate on the same side as the one end of the bottom face of the metal plate, i.e., the length shown by L1 in FIG. 5 (it is assumed that a case where the area of the bottom face is greater than the area of the top face is positive (+)), and this distance will be hereinafter referred to as a xe2x80x9cskirt spreading lengthxe2x80x9d), and the length of a brazing filler metal protruding from the interface of the metal plate and the brazing filler metal bonded thereto (the length shown by L2 in FIG. 5), which will be hereinafter referred to as a xe2x80x9cbrazing filler metal protruding lengthxe2x80x9d, is in the range of from xe2x88x9250 xcexcm to +30 xcexcm. in addition, Japanese Patent 2,797,011 has proposed a substrate having a structure wherein a brazing filler metal protruding length from the interface between a metal plate and a brazing filler metal bonded thereto is 250 xcexcm or more.
However, even if a brazing filler metal protruding length from the interface between a metal plate and a brazing filler metal bonded thereto is in the range of from xe2x88x9250 xcexcm to +30 xcexcm, it is not possible to obtain thermal shock resistance sufficient for market""s demands. If a brazing filler metal protruding length from the interface between a metal plate and a brazing filler metal bonded thereto is 250 xcexcm or more, it is possible to obtain sufficiently high thermal shock resistance. However, in the market trend remarkably emphasizing compactness and flexibility in recent years, if the brazing filler metal protruding length is so long, the outside dimension of a substrate is difficult to be allowable in design, so that it is required to provide thermal shock resistance standing comparison with that in the case of a protruding length of 250 xcexcm even if the outside dimension is smaller.
It is therefore an object of the present invention to eliminate the aforementioned problems and to provide a metal/ceramic bonding article which ensures sufficient thermal shock resistance and has a substrate having a small outside dimension and which has both high reliability and compactness.
In order to accomplish the aforementioned and other objects, the inventors have diligently studied and found that it is possible to provide a metal/ceramic bonding article which ensure sufficient thermal shock resistance and has a substrate having a small outside dimension and which has both high reliability and compactness, by optimally controlling a brazing filler metal protruding length so as to be capable of designing a larger part mounting area. As a result, the inventors have made the present invention.
According to one aspect of the present invention, there is provided a metal/ceramic bonding article comprising: a ceramic substrate; and a metal plate bonded to the ceramic substrate via a brazing filler metal, wherein the brazing filler metal protrudes from the bottom face of the metal plate by a length which is longer than 30 xcexcm and which is 250 xcexcm or less.
According to another aspect of the present invention, there is provided a metal/ceramic bonding article comprising: a ceramic substrate; and a metal plate bonded to the ceramic substrate via a brazing filler metal, wherein the brazing filler metal protrudes from the bottom face of the metal plate by a length which is 25% or more of the thickness of the metal plate.
In each of the above described metal/ceramic bonding articles, the length of the protruding portion of the brazing filler metal may be in the range of from 50 xcexcm to 200 xcexcm. The distance between a plane perpendicular to the principal plane of the metal plate at one end of the bottom face of the metal plate and a plane perpendicular to the principal plane of the metal plate at one end of the top face of the metal plate on the same side as the one end of the bottom face of the metal plate may be 50 xcexcm or less assuming that the distance is positive when the area of the bottom face is greater than the area of the top face. The ceramic substrate may be formed of a material selected from the group consisting of oxides, nitrides and carbides. The metal plate may be formed of a material selected from the group consisting of copper, aluminum, alloys containing copper as a principal component, and alloys containing aluminum as a principal component. The brazing filler metal may contain silver and an active metal. The brazing filler metal may include aluminum. The metal plate and the brazing filler metal may be treated by at least one of nickel plating, nickel alloy plating, gold plating and preservation.